Flow-cooled magnet system

ABSTRACT

A flow-cooled magnet system comprising a magnet for generating a magnetic field in a target region and a cooling system comprising a flow path for a coolant fluid. The flow path has a first part arranged in thermal communication with the magnet and a second part in thermal communication with the target region. Each of the magnet and target region are cooled by the flow of coolant in the first and second parts of the cooling system.

FIELD OF THE INVENTION

The present invention relates to a flow-cooled magnet system.

BACKGROUND TO THE INVENTION

In many applications it is desirable to subject a “target” (includingdevices, other apparatus, an experimental sample or indeed an organism),to a controlled magnetic field. As is known, the strongest magneticfields are at present generated electromagnets, cooled to cryogenictemperatures. Depending upon the application in question, the target mayalso be cooled to cryogenic temperatures (for example in the case ofvery sensitive devices), to an intermediate temperature or indeed thelevel of cooling may be minimal such that the target temperature is thatof approximately ambient temperature.

The cooling systems required for maintaining such magnets at operativetemperatures are often substantial. In addition, in order to cool thetarget, cooling systems of a different nature are used, often because ofthe different temperature requirements, the location of the targetwithin the confines of a cryostat bore and in some cases due to theremovable nature of the target from the system.

Known systems are often bulky and many require the warming of the magnetitself in order to exchange targets. There is therefore a need toimprove upon known systems in order to address such deficiencies.

SUMMARY OF THE INVENTION

In accordance with the present invention we provide a flow-cooled magnetsystem comprising: —

-   -   a magnet for generating a magnetic field in a target region;    -   a cooling system comprising a flow path for a coolant fluid, the        flow path having (i) a first part arranged in thermal        communication with the magnet; and (ii) a second part in thermal        communication with the target region such that each of the        magnet and target region are cooled by the flow of coolant in        the first and second parts of the cooling system.

We have realised that, by appropriate design, a system can be producedin which a common cooling system is used for cooling both the magnet andthe target region. This is achievable by the use of a flow-cooled systemin which a coolant flows between one and the other of the first andsecond parts. Since the first part is in thermal communication with themagnet, which generates the largest heat load, preferably the first partis upstream of the second part. Therefore the coolant fluid passesfirstly through the first part and then moves downstream to cool thesecond part. Although the target region may comprise a detector, orother apparatus/devices, preferably the target region is adapted toretain a sample when in use. Such a sample may be retained within asample holder. This sample holder may or may not also form part of thetarget region.

Typically the magnet is contained within a cryostat. Preferably such acryostat comprises at least one radiation shield. The flow path in thiscase is preferably arranged to further comprise a third part in thermalcommunication with the at least one radiation shield. Thus the coolantfluid is preferably used to cool not only the magnet and the target, butalso the at least one radiation shield. Typically the flow path dividesfrom a common path within which the first part is located into at leasttwo separate paths within one of which is located the second part andanother one of which is located the third part. Thus some of the coolantflows within the second part and some within the third part, whereas allof the coolant flows within the first part.

The third part may also be used to cool further apparatus within thecryostat such as current leads for supplying electrical current to themagnet. It will be appreciated that such a magnet is typically asuperconducting magnet operative at cryogenic temperatures, including 7Kelvin and below, and particularly liquid helium temperatures (4.2Kelvin).

Since not all of the fluid flows through each of the first, second andthird parts, preferably the system further comprises a flow controllerwhich is adapted to provide a predetermined flow rate of coolant withinone or more of the first, second and third parts of the system. The flowcontroller may comprise a variable flow impedance although a staticlocalised flow impedance may be used (such as a narrowed portion ofconduit) where a predefined steady-state flow in each of the parts ofthe system is required.

Typically the second part comprises a heat exchanger with which atarget, such as a sample holder, is in thermal communication. Heatexchange is therefore used to cool the sample via the sample holder. Inorder to provide enhanced control over the temperature within the targetregion, a heater may be provided within the target region such that theheat produced, in combination with the cooling effect of the coolant inthe second part of the cooling system, causes the target region to beheld at a predetermined temperature. The predetermined temperature maytherefore lie in a range bounded by the coolant temperature and ambienttemperature. This allows the target region to be maintained at atemperature substantially the same as that of the magnet itself, oroperated at a higher temperature including room temperature.

In order to provide accurate control over the temperature within thetarget region, preferably the system further comprises a temperaturesensor located within the target region, together with a temperaturecontroller which is used to control the heater in response to signalsreceived from the temperature sensor when in use. If a variable flowimpedance is provided then the temperature controller might be used tocontrol the flow impedance also.

The magnet is preferably located within a cryostat vacuum chamber and,when cooling a sample, the system preferably comprises a sample vacuumchamber in the target region. The sample vacuum chamber is preferablyindependent of the cryostat vacuum chamber and therefore the temperatureand pressure of each of these chambers may be controlled independentlyof one another.

In most cases, the magnet is provided in the form of a solenoid magnethaving a bore within which the target region is located. In someapplications it is desirable to allow radiation (electromagnetic orparticle beam) to be transmitted to and/or received from a sample whenlocated within the target region and therefore in this case the systempreferably further comprises at least a first window position so as toallow radiation to be transmitted and/or received from the sample. Asecond window located on the opposite side of the sample with respect tothe first is also preferably provided so as to allow radiation to bereceived by the sample through the first window, to pass through thesample and then to pass through the second window for monitoring. Thusreflective and transmissive monitoring using electromagnetic radiation(including visible, ultra-violet, infra-red light, and other longer andshorter wavelengths) or particle beams can be achieved.

For such monitoring, the magnet system may be adapted for use upon anoptical bench and is preferably provided with mountings for fitment tosuch an optical bench. In known magnet systems for use upon opticalbenches, a fixed orientation with respect to the bench is required dueto the need for the cooling systems to be arranged in a particularorientation. This is particularly the case where liquid coolants areused within the system, such as when liquid helium is used as a coolant,not only due to its liquid state, but also since it may be within asuperfluid state. In the present case however, the coolant fluid ispreferably within the gaseous form (even if not exclusively so due tolocalised condensation), and the system may be adapted to be operationalin vertical and horizontal orientations with respect to a nominal axis.This is advantageous since it allows greater versatility in terms ofperforming experiments in particular upon target samples. For example amicroscope may be used to view the sample by placing the objective lensinside the bore of the magnet system and then ability to orient thesystem either horizontally or vertically allows greater ease of use of amicroscope or indeed other equipment.

The flow of coolant fluid within the cooling system is preferablyarranged to be continuous such that the system is a “continuous flow”system. However, a discontinuous flow in one or more parts of the systemmay be used in certain circumstances.

Due to the flow of coolant fluid, it is preferable that the magnet iscooled by thermal conduction rather than being placed within a largebath of liquid coolant. The magnet is preferably separated from thecoolant by a high thermal conductivity member through which heat flowsfrom the magnet. To achieve this, the magnet is preferably surrounded byan annular chamber which comprises the first part of the cooling systemthrough which the coolant flows when in use. Thus the surface area ofthe high thermal conductivity member may be maximised so as to produce astrong cooling effect.

The target region may comprise a sample support for receiving a sampleholder within which a sample is retained when in use. This support maybe cooled using a heat exchanger, preferably in combination with aheater and temperature controller as described above.

Whilst, when in use, the coolant preferably flows continually, thesystem may be arranged such that the coolant is supplied to the flowpath from a coolant reservoir. In this case the maximum operationalperiod of the system is determined by the amount of coolant within thereservoir. In an alternative case, the cooling system may be a closedloop system such that, after passing through the flow path, the coolantis refrigerated and recirculated repeatedly through the flow path. Amechanical refrigerator could be used for refrigerating the coolant forrecirculation although it will be appreciated that other cooling systemscould be used to provide a similar effect.

Preferably the coolant is a cryogenic fluid, such fluids including gasesor liquids at cryogenic temperatures (below 100 Kelvin). The coolant maytherefore be helium or nitrogen gas for example.

Some examples of magnet systems according to the present invention willnow be described with reference to the accompanying drawings, in which:—

FIG. 1 is a first example of a flow-cooled system; and,

FIG. 2 is a second example, using recirculated coolant.

DESCRIPTION OF EXAMPLES

With reference to FIG. 1, an example system according to the inventionis generally indicated at 1. The system described within the presentexample is an “MO station” allowing optical monitoring to be performedupon a sample held within a magnetic field.

A cryostat 2 is provided, this containing a main vacuum chamber 3 withinwhich is positioned a magnet 4. The cryostat 2 and magnet 4 are formedin a solenoid configuration so as to provide a bore 5 runningsubstantially centrally through the magnet 4 and cryostat 2.

The magnet 4 is a superconducting solenoid magnet producing a magneticfield strength of about 5 Tesla. The magnet his energised using currentleads 6 (shown as a dashed line). The magnet may operate in a“persistence” mode as is known in the art. The magnet 4 is bounded bywalls 7 having a high thermal conductivity. In this case the walls aremanufactured from high purity copper although of course the magnetwindings are electrically insulated from the walls 7. Around the outerperiphery of the magnet 4, beyond a dividing part of the walls 7, anannular chamber 8 is provided about the magnet periphery. In use coolantgas flows through the annular chamber 8 so as to cool the magnet 4. Inthe present example gaseous helium is used as the coolant. It is notessential that the helium is in its gaseous form to cool the magnet.Indeed the system may be arranged such that the annular chamber 8 isfilled with liquid helium in an alternative embodiment. Gaseous heliumis used in the present example since this allows the magnet to beoriented horizontally or vertically.

Within the bore 5, a sample vacuum chamber 9 is located. This can bethought of a separate cryostat within the cryostat 2. The vacuum chamber9 is generally cylindrical in form such that outer wall of the cylinderconforms with the wall of the bore 5 (although it is separated therefromby a small gap to provide thermal insulation). At least part of theinner wall is formed by a removable sample support 10, this also havinggenerally cylindrical geometry including a central sample support bore11. This bore 11 has a narrower diameter than that of the bore 5. At oneend (the upper end in FIG. 1) of the sample support 10, an alumina(sapphire) sample holder 12 is located. This retains a sample when inuse centrally and at the end of the support bore 11. In the present casethe sample is held at the same pressure as the vacuum chamber 9 withinwhich it is located although this is not essential and the sample can besealed within a dedicated pressure environment.

The sample support 10 is positioned such that the sample holder 12 islocated beneath a first window 13 (glass) in an upper central wall ofthe sample vacuum chamber 9. At the opposite end of the sample support10 to the sample holder 12 the support bore 11 is sealed by a secondwindow 14, also made from glass. The first and second windows 13, 14allow electromagnetic radiation to pass from one side of the cryostatalong the bores 5 and 11, through the sample 12 and to the other side ofthe cryostat. This allows monitoring of the sample in terms of opticalobservations or measurements of the optical behaviour of the samplewithin the sample holder 12. One such application is in making opticalmeasurements upon “quantum dot” samples by external illumination withlight emitting diode or laser devices.

Within the walls of the sample holder 12, a heat exchanger 15 isprovided, this taking the form of a helical flow path formed by opposedwalls cut as a meshing thread with the path defining a narrow helicalgap between the walls. A heater 16 is also provided adjacent to thesample holder 12, together with a temperature sensor (not shown). Thecooling system for the apparatus according to the invention is nowdescribed.

In the present example the coolant is gaseous helium at ambient pressureand at a temperature of approximately 4.2 Kelvin. Lower temperaturehelium could be used (such as 2 Kelvin) although to retain the gaseousstate the system would need to be operated at low pressure. The gaseoushelium is provided from a helium reservoir 20 shown schematically inFIG. 1. The helium is pumped from the reservoir 20 along a supplyconduit 21 and into the cryostat 2 where it opens through a port intothe annular chamber 8 adjacent the conducting walls 7 of the magnet 4.The annular chamber 8 operates as a “first part” of the flow path, thisbeing in thermal communication with the magnet and helium entering theannular chamber therefore cools the magnet by contact with the walls 7.The helium then exits the annular chamber 8 via an exit conduit 22having a port which is distal from the opening of the supply conduitinto the chamber 8. Note in FIG. 1 that the exit and supply conduits areshown one beneath the other only for simplicity purposes within thefigure.

At a branch point 23, downstream of the annular chamber 8, the exitconduit divides into a cryostat conduit 24 and a sample conduit 25.Although the branch point is shown in FIG. 1, it will be appreciatedthat a similar effect can be achieved by connecting the cryostat andsample conduits 24 and 25 to different parts of the annular chamber 8(through respective ports), thereby removing the need for the exitconduit 22 and branch 23.

The purpose of the cryostat conduit is to take some of the coolant gasand use its cooling effect to cool further parts of the cryostatapparatus. In the present case, the cryostat conduit 24 is placed inthermal communication with the cryostat radiation shields generallyindicated at 26. Thereafter, further downstream, the coolant is broughtinto thermal communication with the current leads 6 of the magnet 4, asindicated at 27. In each case suitable heat exchangers may be used tomaximise the cooling effect. Having cooled the current leads 6, thecryostat conduit 24 passes out of the cryostat and through a check valve30. The check valve provides a localised flow impedance and thereforecontrols the relative amounts of fluid flowing in each of the cryostatand sample conduits 24, 25. The check valve is not essential if thenatural flow impedance of the cooling system as a whole is arranged toprovide the correct division of coolant flow rates between the cryostatconduit 24 and sample conduit 25. As is known, the impedance of theconduits is a function of their length and cross section and one or eachof these can be arranged accordingly.

Returning once more to the branch point 23, the sample conduit 25 (whichrepresents a “second part” of the flow path), is arranged to openthrough a port into the helical threaded space defining the heatexchanger 15 within the sample support 10. The helium coolant gastherefore flows through the helical path of the heat exchanger and coolsthe sample support 10 and, by conduction, the sample holder 12. Aventing portion 28 of the sample conduit carries the coolant gas from aport in a distal part of the heat exchanger 15 with respect to theposition of the entry port, out of the cryostat 2 to a venting location.

The sample 12 is therefore cooled by the operation of the heat exchanger15 as a result of the large surface area provided by the helical flowpath. The temperature of the sample 12 is a function of the temperatureand flow rate of the coolant gas, this being controlled at least in partby the check valve 30. In order to provide more accurate control of thetemperature of the sample in the sample holder 12, the heater 16 is usedin conjunction with the cooling effect of the heat exchanger 15 using anappropriate control system and temperature sensor (not shown) so as toensure that the sample is subjected to a predetermined temperature ortemperature cycle. This can be performed using a microprocessor and afeedback loop.

As is illustrated in FIG. 1, the walls of the vacuum chamber 9 and thegeometry of the sample support 10 are such that the first window 13 ispositioned in close proximity to the sample within the sample holder 12.This allows optical equipment such as a microscope to be brought closeto the sample within the sample holder 12. In addition, the windows 13and 14 allow electromagnetic radiation to impinge upon and betransmitted by the sample either in a reflective or transmissive mode.This allows the optical properties of a sample to be measured as afunction of magnetic field in terms of reflection and transmission.

The cooling of the apparatus using the cooling system (comprising theconduits, heat exchangers and so on) will now be described. Helium gasis pumped through the conduit by a pumping system which may be providedeither within a common part of the flow path such as within the supplyconduit 21, or in each of the cryostats and sample conduits 24, 25respectively. As shown in FIG. 1, helium enters the supply conduit 21(denoted by a single headed arrow) and passes into the annular chamber 8of the magnet system. Within the chamber 8, heat exchange occurs withthe walls 7 and therefore the magnet 4 such that the helium absorbs heatfrom the magnet, therefore cooling it. The helium then passes out of theannular chamber 8 and reaches the branching point 23. A portion of thehelium (denoted by doubled headed arrows) passes along the sampleconduit and into the heat exchanger 15 whereby the sample holder 12 iscooled. This raises the temperature of the helium and it then passesbeyond the heat exchanger 15 and vents outsides the cryostat through theventing portion 28. The vented helium may be collected for further useif desired, outside the apparatus.

The conduits 21 and 22 through which all of the coolant flows may bethought of as a common part of the flow cooling system, this containingthe first part in which the thermal communication of the magnet occurs.In the second part, within the sample conduit branch, the thermalcommunication with the sample holder 12 occurs. Part of the coolantbranches along the cryostat conduit 24 and this performs heat exchangewith the radiation shields 26 and the current leads 6 before exiting thecryostat through the check valve 30. The parts of the flow path withinthe cryostat conduit 24 that are in thermal communication with theradiation shields and current leads comprise a third part of the flowpath. As for the venting via the venting portion 28 of the sampleconduit, downstream of the check valve 30, the helium gas may also becollected for further use.

We turn now to a second embodiment of the present invention, this beingillustrated within FIG. 2.

As will be appreciated from a consideration of the example describedabove in association with FIG. 1, the continuous flow operation of theexample there described is limited ultimately by the volume of liquidhelium within the reservoir 20. In the second example now described, arecirculating system is provided this being able to effectively providecontinuous operation with the system for an indefinite period.

Referring now to FIG. 2, in which similar components as to FIG. 1 aredenoted by similar reference numerals, a continuous flow system isdescribed. The second example is denoted by a reference numeral 100.

The reservoir of the first example is replaced in this example by avessel 35 as shown in FIG. 2. The vessel 35 contains a reservoir ofliquid helium although, unlike in the first example, in this case thevessel is cooled by a mechanical refrigerator 36. The mechanicalrefrigerator may take a number of forms that are known in the artalthough in the present case a three-stage pulse tube refrigerator isused. This maintains the vessel 35, and the helium within it, at thedesired operational temperature (about 4.2K for ambient pressure). As isalso illustrated in FIG. 2, a pump 37 is provided within the supplyconduit 21.

In this example, the vessel 35 is sealed from the external environmentand thermally insulated therefrom also, and the pump 37 is operative tosupply gaseous helium along the supply line 21. As before, the valve 30controls the relative flow between the two conduits 24, 25. Unlike inthe previous example however, where each of these conduits was ventedeither to the external atmosphere or to a collection system, in thepresent example the gas is returned to the vessel 35 in each case whereit is cooled again by the mechanical refrigerator 36 which maintains thetemperature of the vessel such that the helium entering the supplyconduit 21 is at a sufficiently low temperature. As in the case of FIG.1, the FIG. 2 example is also extremely schematic.

In a practical system, as would be appreciated by one of ordinary skillin the art, greater thermal efficiency can be achieved by techniquessuch as using the cooling effect of the helium within the “exit” path soas precool the “entry” path of the gas flowing into the cryostat. Thismay be achieved for example by providing the venting portion 28 in FIG.1 along a similar path to the supply conduit 21 and in thermalcommunication therewith.

The abovedescribed examples have been discussed in connection with theuse of helium, this is due to the present desired use of liquid heliumfor cooling superconducting magnets and performing certain lowtemperature experiments. However, the system may also be used with othercryogenic coolants such as nitrogen at for example 77 Kelvin rather than4.2 Kelvin. The systems are also described using approximately ambientpressures, whereas it will be appreciated that by using non-ambientpressures within the cooling system, different cooling temperatures maybe achieved. Whilst the apparatus described in the examples is inrelation to an experimental magnet for optical experiments, it will beappreciated that the principle underlying how the system is cooled maybe applied to magnet systems including nuclear magnetic residence andmagnetic residence imaging systems.

The present examples are particularly useful for performingmagneto-optical experiments on an optical bench and for this reason ineach example, as is shown in FIG. 1, mountings 40 may be provided suchthat the system may be attached to an optical bench. In FIG. 1 the bore5 is oriented substantially vertically. Since the examples describedherein use gaseous helium and therefore there is no or at least littleliquid present within the cryostat itself, the cryostat may therefore bemounted such that the bore is oriented substantially horizontally whichis particularly useful in the event if experiments are to be performedfor example involving laser beams passing along an optical bench.

1. A flow-cooled magnet system comprising: — a magnet for generating amagnetic field in a target region; a cooling system comprising a flowpath for a coolant fluid, the flow path having (i) a first part arrangedin thermal communication with the magnet; and (ii) a second part inthermal communication with the target region such that each of themagnet and target region are cooled by the flow of coolant in the firstand second parts of the cooling system.
 2. A magnet system according toclaim 1, wherein the first part is upstream of the second part.
 3. Amagnet system according to claim 1, wherein the target region is adaptedto retain a sample when in use.
 4. A magnet system according to claim 1,wherein the magnet is contained within a cryostat comprising at leastone radiation shield, wherein the flow path further comprises a thirdpart in thermal communication with the at least one radiation shield,and wherein the flow path divides from a common path within which thefirst part is located, into at least two separate paths, within one ofwhich is located the second part and another one of which is located thethird part.
 5. A magnet system according to claim 4, wherein the flowpath is divided such that some of the coolant flows through the secondpart and some through the third part.
 6. A magnet system according toclaim 4, further comprising current leads for supplying electricalcurrent to the magnet, and wherein the third part is in thermalcommunication with the current leads so as to cool the current leadswhen in use.
 7. A magnet system according to claim 4, further comprisinga flow controller adapted to provide a predetermined flow of coolantwithin one or more of the first, second and third parts of the system.8. A magnet system according to claim 7, wherein the flow controllercomprises a variable flow impedance or a static localised flowimpedance.
 9. A magnet system according to claim 1, wherein the secondpart comprises a heat exchanger with which a sample is in thermalcommunication.
 10. A magnet system according to claim 1, wherein aheater is provided within the target region such that the heat produced,in combination with the cooling effect of the coolant in the second partof the cooling system, allows the target region to be held at apredetermined temperature.
 11. A magnet system according to claim 10,wherein the predetermined temperature lies in a range from the coolanttemperature to ambient temperature.
 12. A magnet system according toclaim 9, wherein the target region further comprises a temperaturesensor and a temperature controller is provided for controlling theheater in response to signals received from the temperature sensor. 13.A magnet system according to claim 1, wherein the magnet is locatedwithin a cryostat having a vacuum chamber, and wherein a sample vacuumchamber is provided as a target region for containing the second part ofthe coolant system and, when in use, a sample to be monitored, andwherein the sample vacuum chamber is independent of the cryostat vacuumchamber.
 14. A magnet system according to claim 13, wherein thetemperature and pressure of the sample vacuum chamber are eachcontrollable independently of the temperature and pressure within thecryostat vacuum chamber.
 15. A magnet system according to claim 1,wherein the magnet is a solenoid magnet having a bore and wherein thetarget region is located within the bore.
 16. A magnet system accordingto claim 1, further comprising at least a first window positioned toallow electromagnetic radiation to be transmitted and/or received from asample when located in the target region.
 17. A magnet system accordingto claim 16, further comprising a second window located on the oppositeside of the sample with respect to the first window so as to allowelectromagnetic radiation to be received by the sample through the firstwindow, to pass through the sample and then pass through the secondwindow for monitoring.
 18. A magnet system according to claim 1, furthercomprising mountings for fitment of the system to an optical bench. 19.A magnet system according to claim 1, wherein the system is defined by anominal axis and the system is adapted to be operational with the axisorientated substantially vertically and substantially horizontally. 20.A magnet system according to claim 1, wherein the system is a continuousflow system adapted such that the coolant flows through the coolingsystem continually when in use.
 21. A magnet system according to claim1, wherein the magnet is cooled by thermal conduction.
 22. A magnetsystem according to claim 21, wherein the magnet is separated from thecoolant by a high thermal conductivity member through which heat flowsfrom the magnet.
 23. A magnet system according to claim 22, wherein themagnet is surrounded by an annular chamber comprising the first part ofthe cooling system through which the coolant flows when in use.
 24. Amagnet system according to claim 1, wherein the target region furthercomprises a sample support for receiving a sample holder within which asample is retained.
 25. A magnet system according claim 1, wherein thecoolant is supplied to the flow path from a coolant reservoir.
 26. Amagnet system according to claim 1, wherein the cooling system is aclosed loop system such that, after passing through the flow path, thecoolant is refrigerated and recirculated repeatedly through the flowpath.
 27. A magnet system according to claim 26, further comprising amechanical refrigerator for refrigerating the coolant for recirculation.28. A magnet system according to claim 1, wherein the coolant is acryogenic fluid.
 29. A magnet system according to claim 28, wherein thecoolant is a gas.
 30. A magnet system according to claim 29, wherein thecoolant is helium or nitrogen gas.